There are many different forms of prior art zener diode construction and each one attempts to achieve improved circuit characteristics including the following general classes. A sharp reference voltage breakdown is desired as the prime function of the zener diode. This reference voltage level should be predictable, and should be nearly independent of diode current. A zener diode should have a low noise performance characteristic which minimizes the broad band noise normally generated by a zener diode. This broad band noise is detrimental to the operation of the circuit because many times it is coupled to the output and provides a noise characteristic at the output terminals. A zener diode for special circuit operations should be bilaterally conducting insofar as it not only operates in the reverse breakdown mode for generating a reference voltage, but it also operates as a forward biased rectifying diode. Most prior art zener diodes which operated in the forward biased direction possessed an extremely low current transfer efficiency from the anode to the cathode. In the prior art devices, this transfer efficiency was no greater than 10 percent. The current flowing from the anode to the cathode escapes to the substrate and is not collected by the cathode.
An additional characteristic of a zener diode is that the required reverse breakdown current drawn in the DC state should be kept as low as possible so as to reduce the standby current drain from the battery when the zener diode is operating in an automobile within the battery regulator. In all other environments, a low standby current reduces temperature of the device by reducing power dissipation in the device.
In the prior art, an NPN transistor is used as a zener diode in order to achieve bilateral conduction. However, such an NPN transistor uses the emitter-base junction operated in the reverse breakdown mode as the zener junction. Such a device, when operating as a zener diode, has two basic drawbacks. First, to achieve the low noise and provide a solid reference voltage, the reverse breakdown current must be very large. These large currents are measured in the milliamps and such a current creates a temperature problem on an IC chip which must dissipate heat generated in the zener diode. When the current flow is reduced to avoid the temperature problems, the reference voltage level is no longer predictable, but rather, it varied within the range of ten percent of its design value. A low current flow of 100 microamps in an NPN transistor still resulted in a 3 millivolt RMS noise signal, as the typical noise signal, at the output terminal. Accordingly, such an NPN transistor connected as a zener diode provides a poor reference voltage standard as well as provides a high noise term on the reference voltage.
The prior art also teaches the use of a zener diode using the finger emitter design for achieving high current density with a low reverse current. While this construction gives a low noise at a reasonably low current, it is a poor device for operating in the bilateral conduction mode. The transfer efficiency from the anode to the cathode is again less than 10 percent.